JComponents没有显示图片背景？

Question

我的组件没有显示出来。我该如何解决这个问题？

代码：

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.*;

public class login implements ActionListener{
    JTextField gusername;
    JTextField gpassword;
    static String username;
    static String password;

    void logini() throws IOException {
        JFrame window = new JFrame("Login");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setSize(300, 250);
        window.setResizable(false);
        window.setVisible(true);

        JPanel mainp = new JPanel(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        window.add(mainp);

        BufferedImage myPicture = ImageIO.read(new File("c:\\bgd.png"));
        JLabel picLabel = new JLabel(new ImageIcon( myPicture ));
        mainp.add(picLabel, c);

        c.gridx = 0;
        c.gridy = 1;
        gusername = new JTextField();
        gusername.setText("Username");
        mainp.add(gusername, c);

        c.gridx = 0;
        c.gridy = 2;
        gpassword = new JTextField();
        gpassword.setText(" password ");
        mainp.add(gpassword, c);

        c.gridx = 0;
        c.gridy = 3;
        JButton login = new JButton("Login");
        mainp.add(login, c);

        login.addActionListener(this);
        login.setActionCommand("ok");
    }

    public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equalsIgnoreCase("ok")){
            try {
                this.username = (gusername.getText());
                this.password = (gpassword.getText());
                System.out.println("0");
            }
            catch(NumberFormatException ex){
                System.out.println("ERROR: Could not preform function: 7424");
            }
        }
    }
}

结果：

Answer 1

window.setVisible(true); should be invoked only after all the components have been added to the frame.
 void logini() throws IOException {
        JFrame window = new JFrame("Login");
        JPanel mainp = new JPanel(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        window.add(mainp);

        BufferedImage myPicture = ImageIO.read(new File("c:\\bgd.png"));
        JLabel picLabel = new JLabel(new ImageIcon( myPicture ));
        mainp.add(picLabel, c);

        c.gridx = 0;
        c.gridy = 1;
        gusername = new JTextField();
        gusername.setText("Username");
        mainp.add(gusername, c);

        c.gridx = 0;
        c.gridy = 2;
        gpassword = new JTextField();
        gpassword.setText(" password ");
        mainp.add(gpassword, c);

        c.gridx = 0;
        c.gridy = 3;
        JButton login = new JButton("Login");
        mainp.add(login, c);

        login.addActionListener(this);
        login.setActionCommand("ok");

        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setSize(500, 250);
        window.setResizable(false);
        window.setVisible(true);
    }

Answer 2

首先看这个小例子，如果你明白这里发生了什么，请告诉我。那么只有我们会更进一步，慢慢地慢慢地。尝试通过这个示例，我将向您展示如何在 JPanel 上绘图

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.*;

public class PaintingExample {

    private CustomPanel contentPane;

    private void displayGUI() {
        JFrame frame = new JFrame("Painting Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        contentPane = new CustomPanel();        

        frame.setContentPane(contentPane);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String... args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PaintingExample().displayGUI();
            }
        });
    }
}

class CustomPanel extends JPanel {

    private BufferedImage image;

    public CustomPanel() {
        setOpaque(true);
        setBorder(BorderFactory.createLineBorder(Color.BLACK, 5));
        try {
            /*
             * Since Images are Application Resources,
             * it's always best to access them in the
             * form of a URL, instead of File, as you are doing.
             * Uncomment this below line and watch this answer
             * of mine, as to HOW TO ADD IMAGES TO THE PROJECT
             * http://stackoverflow.com/a/9866659/1057230
             * In order to access images with getClass().getResource(path)
             * here your Directory structure has to be like this
             *                 Project
             *                    |
             *         ------------------------
             *         |                      |
             *        bin                    src
             *         |                      |
             *     ---------             .java files             
             *     |       |                   
             *  package   image(folder)
             *  ( or              |
             *   .class        404error.jpg
             *   files, if
             *   no package
             *   exists.)
             */
            //image = ImageIO.read(
            //      getClass().getResource(
            //              "/image/404error.jpg"));
            image = ImageIO.read(new URL(
                        "http://i.imgur.com/8zgHpH8.jpg"));
        } catch(IOException ioe) {
            System.out.println("Unable to fetch image.");
            ioe.printStackTrace();
        }
    }

    /*
     * Make this one customary habbit,
     * of overriding this method, when
     * you extends a JPanel/JComponent,
     * to define it's Preferred Size.
     * Now in this case we want it to be 
     * as big as the Image itself.
     */
    @Override
    public Dimension getPreferredSize() {
        return (new Dimension(image.getWidth(), image.getHeight()));
    }

    /*
     * This is where the actual Painting
     * Code for the JPanel/JComponent
     * goes. Here we will draw the image.
     * Here the first line super.paintComponent(...),
     * means we want the JPanel to be drawn the usual 
     * Java way first (this usually depends on the opaque
     * property of the said JComponent, if it's true, then
     * it becomes the responsibility on the part of the
     * programmer to fill the content area with a fully
     * opaque color. If it is false, then the programmer
     * is free to leave it untouched. So in order to 
     * overcome the hassle assoicated with this contract,
     * super.paintComponent(g) is used, since it adheres
     * to the rules, and performs the same task, depending
     * upon whether the opaque property is true or false),
     * then later on we will add our image to it, by 
     * writing the other line, g.drawImage(...).
     */
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(image, 0, 0, this);
    }
}

编译运行：

1. 启动命令提示符/终端/cmd。并移动到指定的位置 到 Project 文件夹

2. 现在使用以下代码编译代码：

   C:\Project>javac -d bin src\*.java

3. 现在移动到 bin 文件夹，通过发出命令：

   C:\Project>cd bin

4. 进入 bin 文件夹后，发出以下命令运行：

   C:\Project\bin>java 绘画示例

以下是使用JLabel 作为图像基础时的代码：

import java.awt.*;
import java.net.MalformedURLException;
import java.net.URL;
import javax.swing.*;

public class LabelExample {

    private JPanel contentPane;
    private JLabel imageLabel;

    private void displayGUI() {
        JFrame frame = new JFrame("Label Example");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        contentPane = new JPanel();
        contentPane.setOpaque(true);
        contentPane.setBorder(
                BorderFactory.createLineBorder(Color.BLACK, 5));
        //imageLabel = new JLabel(
        //          new ImageIcon(
        //              getClass().getResource(
        //                  "/image/404error.jpg")));
        try {
            imageLabel = new JLabel(new ImageIcon(
                    new URL("http://i.imgur.com/8zgHpH8.jpg")));
        } catch(MalformedURLException mue) {
            System.out.println(
                    "Unable to get Image from"
                        + "the Resource specified.");
            mue.printStackTrace();
        }
        contentPane.add(imageLabel);
        frame.setContentPane(contentPane);  
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String... args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new LabelExample().displayGUI();
            }
        });
    }
}

这是上面两个代码的输出：